Each day a different image or photograph is featured, with an
accompanying caption, which deals with various topics in Earth Science.

Tucson Infralateral Arc and Circumscribed Halo

The photo above features a striking infralateral arc (bottom) and portion of a circumscribed 22 degree halo as observed near Tucson, Arizona on July 15, 2008. I was driving with my little brother (from the Big Brothers program), and he was staring out the passenger window of my car at 12:30 p.m. on a 103 degree (39 C) summer’s day in Tucson. He excitedly yelled that he saw a rainbow, but I dismissed him a bit, figuring he was seeing interference in the window tinting or diffraction off something outside the car. I was eventually able to convince him that he probably didn't see an actual rainbow, but as we made a final turn south into the parking lot of our destination, he said he saw it again. I got out of the car, looked up in the sky and had to apologize to him after I caught my breath. This picture was taken from the shade of a grapefruit tree, so that I could get both the infralateral arc and the circumscribed halo in the frame.

Infralateral arcs occur under similar conditions as circumscribed halos. Both are formed by crystals that are spinning. Ice crystals with a hexagonal column shape that are well aligned form the infralateral arc, while more random orientations create the 22 degree halo. The Sun was at about 80 degrees from the horizon as this picture was taken, and at this angle the circumscribed halo and 22 degree halo nearly overlap. At 68 degrees, the infralateral arcs form a common arc that touches the 46 degree halo directly below the Sun. As the Sun rises higher, the vertex sinks below the 46 degree arc.

Initially I thought that this was a circumhorizontal arc, caused by hexagonal plate crystals instead of the hexagonal column crystals, but the distinct color of the halo at 22 degrees as well as the increased curve away from the horizon at the periphery of the arc indicated to me that this was in all likelihood an infralateral arc.